Fly ash, bark ash, spray dryer ash, bottom ash, bottom slag, boiler slag, and other pozzolanic ashes are typically produced as waste byproducts from certain combustion or chemical processes. Most of these pozzolanic ashes are disposed of in landfills or similar large waste-containment facilities. Increasingly, however, some pozzolanic ashes are being used as a partial replacement for Portland cement in concrete mixtures. By way of example, while Table 1 shows the majority of the 71 million tons of fly ash produced in the United States in 2004 was treated as waste and disposed of in landfills or surface impoundments, Table 1 also shows that approximately 40% of the fly ash was recycled into various applications. Specifically, Table 1 shows that the largest amount of the recycled fly ash was used as a partial replacement for Portland cement in concrete and grout.
TABLE 1Fate of fly ash from coal combustion in the United States in 2004FateMillions of tons%Landfill disposal42.760Concrete/grout14.520Structural fills4.76.6Waste stabilization2.43.4Mining applications1.11.5Raw feed for cement kiln2.33.2Road base/sub-base0.71.0Flowable fill0.81.1Other1.82.5Total71100
Recent goals, set by several fly-ash-producing industries and the federal government, seek for increased use of fly ash (e.g., 50% utilization of fly ash by 2011) and other pozzolanic ashes. However, under certain current methods, concrete requires Portland cement as a cementitious binder and the amount of fly ash that can be used in the concrete is often limited.
One of the main reasons that only a small amount of fly ash is suitable for use as a concrete additive is that fly ash, which contains carbon, may absorb costly air-entraining agents (AEAs), which are added to concrete in order to improve its workability and resistance towards freeze-thaw damage. When carbon absorbs air-entraining agents, the agents become less available to entrain tiny air bubbles in the concrete that give concrete its protection against freeze-thaw conditions. Because the carbon content of fly ash can vary widely depending upon the fuel source and boiler-burn conditions, some fly ashes are better than others for use in concrete.
Where fly ash is used to fabricate concrete, the fly-ash-containing concrete is often made to be impervious to water. As a result, water is often unable to drain through the impervious concrete to replenish the water table, and the water is directed to storm drains or retention ponds. This implies that traditional concrete can increase stormwater runoff. Similarly, because impervious concrete may not allow water to seep directly through it into the ground, such concrete may increase chances of flooding and prevent urban trees and vegetation from having viable rooting space.
In light of the aforementioned discussion, it would be an improvement in the art to provide an environmentally favorable use for pozzolanic ashes, such as fly ash, that would otherwise be disposed of in landfills or in large waste containment facilities. Additionally, it would be an improvement to provide concrete containing pozzolanic ashes, wherein the concrete is pervious to water.